In recent years, in hydraulic drive systems for a construction machine such as a hydraulic excavator, a hydraulic crane and the like, which comprise a plurality of hydraulic actuators for driving a plurality of driven units, delivery pressure of the hydraulic pump is controlled in synchronism with load pressure or requisite flow rate. Further a plurality of pressure compensating valves are arranged respectively in association with the flow control valves for controlling differential pressure across the flow control valves, whereby supply flow rates during simultaneous driving of the actuators are stably controlled. Of these hydraulic drive systems, load-sensing control is known from DE-A1-3422165 (corres. to JP-A-60-11706), U.S. Pat. No. 4,739,617 and the like, a typical example of which is the control of delivery pressure of the hydraulic pump in synchronism with load pressure. The load-sensing control is such that pump delivery rate is controlled so as to make the pump delivery pressure higher by a fixed value than the maximum load pressure among a plurality of hydraulic actuators. In these conventional examples, a swash-plate position of the hydraulic pump is controlled in response to the differential pressure between the delivery pressure of the hydraulic pump and the maximum load pressure among the plurality of actuators, to conduct the load-sensing control.
Further, in these conventional systems, when the delivery rate of the hydraulic pump reaches its maximum so that the pump delivery rate is insufficient, the hydraulic fluid is preferentially supplied to the actuator on the side of the low load pressure during the combined operation. Thus, balance of the combined operation cannot be maintained. In order to solve this problem, a control force determined on the basis of the differential pressure between the delivery pressure of the hydraulic pump and the maximum load pressure of the plurality of actuators acts directly or indirectly upon each pressure compensating valve for controlling the differential pressure across the flow control valve, in place of a spring as one for setting a target value of the differential pressure. In this arrangement, the target value of the differential pressure across the flow control valve decreases in response to a decrease in the differential pressure between the pump delivery pressure and the maximum load pressure. The pump delivery rate is accordingly distributed in response the opening ratio (requisite flow-rate ratio) of the flow control valves. Thus, it is possible to maintain the balance of the combined operation.
Additionally, the hydraulic pump is driven by the prime mover. The delivery rate of the hydraulic pump is represented by the product of a displacement volume determined by the swash-plate tilting angle of the hydraulic pump and the rotational speed of the prime mover. The pump delivery rate decreases when the target rotational speed of the prime mover decreases. Further, in the conventional systems described above, a change in the passing flow rate of each of the flow control valves, with respect to a change in a stroke of a control lever, is constant regardless of the target rotational speed of the prime mover. Accordingly, in these conventional systems, when the pump delivery rate, at the time the target rotational speed of the prime mover decreases and the displacement volume is maximum, is reduced less than the requisite flow rate at the time the opening of the flow control valve is maximum, the following result occurs. Specifically, the passing flow rate, that is the flow rate supplied to the actuators, reaches its maximum before the opening of the flow control valve reaches its maximum when the stroke of the control lever increases, so that a range capable of controlling the supply flow rate in accordance with the stroke of the control lever, that is, a metering range of the control lever stroke, is shortened. This means that the metering range varies dependent upon a change in the target rotational speed. Thus, an operator perceives a problem of operability.
Further, in the hydraulic excavator, when a precise operation such as a leveling orthopedic operation is conducted, the target rotational speed of the prime mover is frequently reduced to decrease the pump delivery rate. When the target rotational speed is reduced, however, the metering range decreases correspondingly and, further, even if the target rotational speed is reduced, a change in the passing flow rate of the flow control valve with respect to a change in the control lever stroke is constant. Accordingly, the control of the supply flow rate must be conducted at the same rate as the case of the ordinal or usual operation within the small metering range. Thus, there is a problem that the precise operation is difficult.
Moreover, assuming that there are a flow control valve that is relatively small in maximum opening, and a flow control valve that is relatively large in the maximum opening, when the target rotational speed of the prime mover is reduced, the flow rate demanded by the maximum opening of the former flow control valve is smaller than the pump delivery rate, and the flow rate demanded by the maximum opening of the latter flow control valve is larger than the pump delivery rate. Then, at the single operation which drives only the former flow control valve, it is possible to obtain the flow rate required by its maximum opening, while the pump delivery rate is insufficient at the combined operation which operates the two flow control valves simultaneously. Accordingly, the pump delivery rate is distributed in accordance with the opening ratio (requisite flow-rate ratio) of the flow control valve by the aforesaid control, and the passing flow rate of the flow control valve used in the actuator of small capacity is considerably reduced as compared with the above-mentioned single operation. In addition, when the target rotational speed of the prime mover is reduced, the pump delivery rate is made insufficient when the flow control valve that is relatively large in maximum opening is driven singly. Accordingly, the passing flow-rate ratio when the two flow control valves are singly driven respectively, and the passing flow-rate ratio in case of the combined operation are not the same as each other. From this, when the rotational speed of the prime mover is reduced to conduct the combined operation, the operator perceives an operability problem.
It is an object of the invention to provide a hydraulic drive system capable of maintaining a metering range of flow control valves substantially constant regardless of a change in target rotational speed of a prime mover.
It is another object of the invention to provide a hydraulic drive system capable of improving an operability perception when target rotational speed of a prime mover decreases.